1. Field of the Invention
This invention relates to a control system for an automatic vehicle transmission.
2. Description of the Related Art
As a typical prior-art control system for automatic vehicle transmissions, Japanese Laid-Open Patent Application No. Hei 6 (1994)-207660 teaches detecting the actual starting point of shift in the transmission precisely and based thereon, controlling the operation of the transmission or the engine so as to decrease shift shock experienced by the vehicle operator. Specifically, in the control, the hydraulic pressure is raised when upshifting until the drive force becomes equal to that generated at the current gear (being engaged) and is then kept for a predetermined time. With this, the prior art system aims to suppress the heating of the clutch and to effect an improved shift shock reduction. However, the drive force acting about the vehicle drive shaft is not the same as the acceleration of gravity acting on the entire vehicle in the linear direction (the longitudinal direction of the vehicle) and in the direction of gravity. As a result, by controlling drive force to that of the current gear, the shock of the entire vehicle may sometimes increase, contrary to what is expected, since, depending upon the operating conditions of the vehicle, the increased torque, raised from a dropped level in the torque-phase, may occasionally generate acceleration in the linear direction of the vehicle and in the direction of gravity. The latter acceleration in the direction of gravity causes the vehicle to pitch, which increases the shift shock contrary to what is expected.
Further, in the prior art system, when the hydraulic pressure is raised until the drive force becomes equal to that at the current gear now being engaged, depending on the change in torque due to the engine speed change before and after the shift, the clutch torque may disadvantageously balance with the engine torque. If this happens, it becomes impossible to finish the shift within an expected period of time.
Furthermore, conventionally, as taught in the prior art, the engine toque is estimated based on the vehicle speed and the throttle opening. Alternatively, it is estimated from information including the engine speed and manifold absolute pressure or from the state of the torque converter, etc.
However, when the engine torque is estimated from the throttle opening, etc., the estimation is likely to be affected by the change in environment. When it is estimated from the manifold absolute pressure, etc., since factors of the torque converter and inertia energy are not taken into account, the estimation accuracy is not always satisfactory. Further, when it is estimated from the state of the torque converter, since the toque absorption characteristic of the torque converter changes suddenly when fully-locked up or thereabout, the estimation accuracy is liable to be degraded particularly in a transient state.
An object of this invention is therefore to overcome the aforesaid problems and to provide a control system for automatic vehicle transmission, which determines the manipulated variables to be supplied to frictional engaging elements such as clutches based on desired value of the acceleration and other parameters including input torque, thereby decreasing the shift shock effectively so as to improve the feeling of the vehicle occupant, while ensuring to finish the shift within an expected period of time.
Another object of this invention is therefore to overcome the aforesaid problems and to provide a control system for automatic vehicle transmission, which calculates the engine torque based on at the engine speed and engine load, with use of a parameter indicative of inertia torque used for raising the engine speed, thereby decreasing the shift shock more effectively.
In order to achieve the objects, there is provided a system for controlling an automatic transmission of a vehicle having an input shaft connected to an internal combustion engine mounted on the vehicle and an output shaft connected to driven wheels of the vehicle, the transmission transmitting input torque, through any of frictional engaging elements, generated by the engine and inputted by the input shaft to the driven wheels by the output shaft, in accordance with predetermined shift scheduling defining a target gear based on detected operating conditions of the vehicle and the engine, comprising; input torque calculating means for calculating the input torque inputted to the transmission; desired torque calculating means for calculating a desired torque, at inertia-phase of shift, of one of the frictional engaging elements of the target gear to be shifted to, based on at least the calculated input torque and a predetermined value selected in a range of acceleration of gravity defined by upper and lower limits when defining the acceleration of gravity at a current gear now being engaged and the target gear as the upper and lower limits; hydraulic pressure calculating means for calculating hydraulic pressure to be supplied to the one of the frictional engaging elements based on at least the calculated desired torque; and hydraulic pressure control circuit for supplying hydraulic pressure to the one of the frictional engaging elements based on at least the calculated hydraulic pressure.